1. Field of the invention
The present invention relates to an endoscope system for applying two or more types of color light sequentially and calculating an oxygen saturation level of hemoglobin in blood and imaging an oxygen saturation level calculated, a processor device for an endoscope system, and an image producing method.
2. Description Related to the Prior Art
Recently in the medical field, diagnoses using endoscope systems have been common. The endoscope system is composed of a light source device, a processor device, and an endoscope device. Normal observation and special observation using the endoscope system are known. In the normal observation, broadband light, for example, white light is used to illuminate a region of interest in a subject. This is effective in observing the region of interest as a whole. In the special observation, the illumination light of particular wavelength(s) is used in accordance with the purpose of the observation.
In the normal observation, the broadband light such as xenon light from the light source device is applied to the region of interest through a lighting section of the endoscope device. A reflection image is imaged with a color image sensor. An image signal acquired is sent to the processor device. Thereby, a color normal image is displayed on a monitor of the processor device.
In the special observation, a blood vessel emphasized image and an oxygen saturation image are observed. This enables finding cancer which is difficult to find in the normal observation. To observe the blood vessel emphasized image, the illumination light in a wavelength range in which an absorption coefficient of hemoglobin in blood is high is used to display an image with the blood vessels of a specific depth emphasized on the monitor. To observe the oxygen saturation image, an oxygen saturation level is calculated using the illumination light in a wavelength range in which absorbance varies depending on an amount of the hemoglobin in blood. The oxygen saturation level is imaged and displayed on the monitor.
The observation of the oxygen saturation level is considered to be effective in finding the cancer because cancer tissue is in a hypoxic or low-oxygen state. For example, in U.S. patent application Publication No. 2011/0237884 (corresponding to Japanese Patent Laid-Open Publication No. 2011-194151), three types of narrowband light (with the peak wavelengths of 405 nm, 440 nm, and 560 nm, respectively) in a blue band or three types of narrowband light (with the peak wavelengths of 540 nm, 560 nm, and 580 nm, respectively) in a green band are used. The image sensor images the region of interest while the three types of narrowband light are emitted sequentially toward the region of interest on a frame-by-frame basis. Thus, three frames of image data are acquired.
There are time lags between the acquisitions of the three frames of image data, resulting in positional shifts among the images of the three respective frames. To solve the problem, a means for extracting blood vessels is used to extract corresponding blood vessels in the respective images, and shift amounts of the images are calculated to align the corresponding blood vessels with each other. One of images of the three frames is moved by the shift amount relative to the image of the reference frame, and the remaining image is also moved by the shift amount relative to the image of the reference frame. Thus, the images of the three frames are aligned with each other. Thereafter, an oxygen saturation level of the blood vessel is calculated on a pixel-by-pixel basis from the image data of the three frames. Then, an oxygen saturation image representing distribution of the oxygen saturation levels is produced and displayed in pseudo-color on the monitor.
Absorption characteristics of hemoglobin and scattering characteristics of digestive tract mucosa significantly vary depending on a wavelength band. Hence, shapes of the blood vessels observed with the three respective types of wavelengths are different from each other. In the U.S. patent application Publication No. 2011/0237884, it is difficult to perform accurate registration or alignment of images because the shift amount between the images of different wavelengths is calculated.
The blood vessels are classified into surface blood vessels and subsurface (medium-depth and deep) blood vessels according to their depth from the surface. It is necessary to calculate the oxygen saturation levels of the blood vessels including both the surface and subsurface blood vessels. To detect the surface blood vessels, the narrowband light in a blue wavelength range is effective. To detect the medium-depth blood vessels, the narrowband light in a green wavelength range is effective. To detect the deep blood vessels, the narrowband light in a red wavelength range is effective. Accordingly, a blue image, a green image, and a red image are necessary to calculate the oxygen saturation levels of the surface and subsurface blood vessels. In the blue image, the surface blood vessels are emphasized while the subsurface blood vessels are inconspicuous. In the red image, on the contrary, the subsurface blood vessels are emphasized while the surface blood vessels are inconspicuous. To register or align the images of the respective frames with each other, the shift amount is calculated from the positions of the corresponding blood vessels in the blue and red images, for example. In this case, the image with the surface blood vessels emphasized is compared with the image with the inconspicuous surface blood vessels. As a result, the shift amount cannot be calculated accurately, making the registration difficult.